1. Field of the Invention
The present invention relates to optical communications and, more particularly, to an erbium doped fiber amplifier (EDFA) having a different structure of an optical portion using an excitation light source for amplifying an input light signal.
2. Description of the Related Art
Generally speaking, when an electric signal is converted into a light signal at the transmission stage and sent to an intended place via fiber optics, that is, a transmission medium, the EDFA amplifies the light signal weakened by a predetermined distance so as to transmit it as a stable signal. The EDFA is installed at the transmission/reception stage for the purpose ofpower amplification and pre-amplification. In an earlier single pumping amplifier, the input connector connects an external fiber optic cable to a internal fiber optic cable of the EDFA. A separation tap for separating a light signal input via the fiber optic cable connected by the input connector at a predetermined ratio splits the input light signal, and inputs the split signals to a photodiode and an optical isolator. Here, the photodiode monitors the magnitude of the light signal input. The optical isolator has one input terminal and one output terminal so that it passes the light signal proceeding to the output terminal from the input terminal within a predetermined wavelength, and interrupts the light signal returning back from the output terminal to the input terminal. By doing so, the optical isolator interrupts the reverse flow of amplified spontaneous emission (ASE) generated from behind erbium doped fiber (EDF) so as to prevent the input light signal from being distorted. The light signal output from the optical isolator is input to a wavelength division multiplexer (WDM). The WDM receives two different wavelengths of light signal via two different input ports, and outputs them via one fiber optic end. Here, the wavelength of the light signal input is 1,550 nm, the wavelength of the excitation light source being 980 or 1,480 nm. Via the output terminal of the WDM, the excitation light source of a wavelength of 980nm and the input light signal of 1,550 nm are fed to the EDF 16. The EDF is made with erbium (element number 68), a rare-earth metal, added to a fiber optic cable, and having a high absorption rate at specific wavelengths such as 800, 980 and 1,480 nm. It amplifies the input light signal, having a spectrum which diverges with a bandwidth of about 30 nm at a predetermined wavelength (1,550 nm). The output end of the EDF is connected to an second optical isolator which is, then connected to a second tap. The tap is connected to the output stage fiber optic cable by the output connector. Here, the second optical isolator interrupts the light signal reflected back from the tap or the connection of the output connector. The second tap receives the light signal output from the second optical isolator, and splits it into a light signal output to the fiber optic cable connected via the output connector and a light signal for monitoring the output light signal. The monitoring light signal is monitored by a second photodiode. For the single pumping amplifier, two kinds of method are used which include a forward pumping structure for supplying a light source of an excitation wavelength by the WDM at the front stage of the EDF, and a reverse pumping structure for supplying the light source of an excitation wavelength by the WDM behind the EDF. Here, for the EDFA, the forward pumping structure uses a pump light of 980 nm with high gain and low noise, and is usually utilized for a preamplifier placed just preceding the reception stage in a communication means. The reverse pumping structure is mostly used for a power amplifier placed just preceding the transmission stage in the communication means because it amplifies a large signal to increase saturation output, using reverse ASE whose intensity is relatively large compared with forward ASE. In order to increase the magnitude of output power and gain, the two kinds of single pumping structure are mixed, as a double pumping structure. However, the double pumping amplifier used to increase output power needs two expensive pump laser diodes, increasing the overall cost of the EDFA. In case where the two pump laser diodes are used at the same time, their service life becomes shorter than when they are used alternately. Furthermore, if they are used in high-speed transmission network through the earlier EDFA, the gain of the transmitted light signal is reduced due to dispersion loss.
The Shibuy patent, U.S. Pat. No. 5,561,552, entitled Optical Fiber Amplifier Unit And Method For Supplying Excited Light Thereof, illustrates an optical fiber amplifier utilizing a pair of laser diode sources 5 and 6 which are affixed to a coupler having outputs fed to WDMs located on either side of the fiber amplifier which is split by an isolator disposed therebetween.
The Hadjifotiou patent, U.S. Pat. No. 5,355,248, entitled Optical Amplifier, illustrates an fiber amplifier 19 having power supplied from a pair of laser pump diodes 10 and 11 through couplers 13, 14, 16 and 17.
The following additional patents each disclose features in common with the present invention but are not as pertinent as the two patents discussed in detail above: U.S. Pat. No. 5,455,704 to Mizuochi et al, entitled Optical-Fiber Light Amplifier, U.S. Pat. No. 5,506,723 to Junginger, entitled Multistage Fiber-Optic Amplifier, U.S. Pat. No. 5,563,732 to Erdoganetal., entitled Laser Pumping Of Erbium Amplifier, U.S. Pat. No. 5,563,733 to Mitsuda et aL, entitled Optical Fiber Amplifier And Optical Fiber Transmission System, U.S. Pat. No. 5,623,362 to Mitsuda et al., entitled Erbium-Doped Fiber Amplifier And An Optical Fiber Communication System, U.S. Pat. No. 5,191,628 to Byron, entitled Optical amplifiers, U.S. Pat. No. 5,185,826 to Delavaux, entitled Hybrid Pumping Arrangement For Doped Fiber Amplifiers, U.S. Pat. No. 5,299,055 to Yoneyama, entitled Optical Fiber Amplifier Circuit Comprising A Control CircuitFor Controlling A Plurality Of Excitation Light Sources, U.S. Pat. No. 5,400,166 to Huber, entitled Communication Of Signals Sharing A single Optical Source, U.S. Pat. No. 5,510,926 to Bayart et al., entitled Transmission Method And An Optical Link Using Multiplexing With Application, U.S. Pat. No. 5,546,221 to Harasawa, entitled Optical Amplifier Apparatus, U.S. Pat. No. 5,598,491 to Ohya et al., entitled Optical fiber Amplifier And Optical Fiber Transmission Apparatus, U.S. Pat. No. 5,475,529 to Takatu et al., entitled Optical Amplifier, U.S. Pat. No. 5,430,572 to DiGiovanni et al., entitled High Power, High Gain, Low Noise, Two-Stage Optical Amplifier, and U.S. Pat. No. 54,57,568 to Jacobovitz Vesslka et al, entitled 980 NM Pumped Erbium Fiber Amplifier.